This proposed investigation will test the hypothesis that, together with Grb2 (Growth factor receptor bound protein- 2), PNRC (Proline-rich Nuclear Receptor Co-regulatory protein) modulates both the nuclear receptor-mediated regulation and the growth factor/Ras mediated regulation in human breast epithelial cells. PNRC was identified in a yeast two-hybrid screening of a human mammary gland cDNA expression library by using bovine SF1 (Steroidogenic Factor 1) as bait. Three important sets of results have been generated in our laboratory. First, by using RT-PCR, PNRC was found to be selectively expressed in breast epithelial cells, and the expression levels in non-cancerous cell lines were found to be higher than those in breast cancer cell lines. Second, PNRC was shown to act as a coactivator for nuclear receptors such as ER (estrogen receptor) and PR (progesterone receptor). The coactivator activity can be suppressed by Grb2. Third, by interacting with Grb2, PNRC suppresses growth factor-induced MAP (mitogen-activated protein) kinase activation. There are five specific aims in this proposal. Aim 1 will study the function of PNRC by stable transfection experiments. Aim 2 will study the dynamics of intracellular movement of PNRC and Grb2 with experiments using transfected fluorescent protein (FP)-PNRC and FP-Grb2 fusion proteins. Aim 3 will determine the molecular basis of the interaction between PNRC and nuclear receptors by protein deletion and site-directed mutagenesis experiments. Aim 4 will determine the molecular basis of the interaction between PNRC and Grb2. Aim 5 will apply the yeast two-hybrid screening method to identify protein(s) in human breast tissue that interacts with PNRC. These experiments will determine the molecular basis of the interaction between PNRC (PNRC2) and Grb2, examine the functions of these proteins on the nuclear receptor-mediated pathway and the growth factor-mediated pathway in human breast, and identify additional molecules in breast tissue that interact with PNRC. This laboratory is familiar with all the experimental techniques proposed in this application. Both the nuclear receptor-signaling pathway and the growth factor-signaling pathway have been extensively studied, and the control mechanisms for these pathways are complex. However, the findings on PNRC and Grb2 represent a newly identified mechanism for modulating these pathways.